LTC3703 datasheet(20/32 Pages) LINER

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LTC3703

3703f

Feedback Component Selection

Selecting the R and C values for a typical Type 2 or Type 3

loop is a nontrivial task. The applications shown in this

data sheet show typical values, optimized for the power

components shown. They should give acceptable perfor-

mance with similar power components, but can be way off

if even one major power component is changed signifi-

cantly. Applications that require optimized transient re-

sponse will require recalculation of the compensation

values specifically for the circuit in question. The underly-

ing mathematics are complex, but the component values

can be calculated in a straightforward manner if we know

the gain and phase of the modulator at the crossover

frequency.

Modulator gain and phase can be measured directly from

a breadboard or can be simulated if the appropriate

parasitic values are known. Measurement will give more

accurate results, but simulation can often get close enough

to give a working system. To measure the modulator gain

and phase directly, wire up a breadboard with an LTC3703

and the actual MOSFETs, inductor and input and output

capacitors that the final design will use. This breadboard

should use appropriate construction techniques for high

speed analog circuitry: bypass capacitors located close to

the LTC3703, no long wires connecting components,

appropriately sized ground returns, etc. Wire the feedback

amplifier as a simple Type 1 loop, with a 10k resistor from

VOUT to FB and a 0.1µF feedback capacitor from COMP to

FB. Choose the bias resistor (RB) as required to set the

desired output voltage. Disconnect RB from ground and

connect it to a signal generator or to the source output of

a network analyzer to inject a test signal into the loop.

Measure the gain and phase from the COMP pin to the

output node at the positive terminal of the output capaci-

tor. Make sure the analyzer’s input is AC coupled so that

the DC voltages present at both the COMP and VOUT nodes

don’t corrupt the measurements or damage the analyzer.

If breadboard measurement is not practical, a SPICE

simulation can be used to generate approximate gain/

phase curves. Plug the expected capacitor, inductor and

MOSFET values into the following SPICE deck and gener-

ate an AC plot of V(VOUT )/V(COMP) in dB and phase of

VOUT in degrees. Refer to your SPICE manual for details of

how to generate this plot.

APPLICATIO S I FOR ATIO

*3703 modulator gain/phase

*2003 Linear Technology

*this file written to run with PSpice 8.0

*may require modifications for other

SPICE simulators

*MOSFETs

rfet mod sw 0.02

;MOSFET rdson

*inductor

lext sw out1 10u

;inductor value

rl out1 out 0.015

;inductor series R

*output cap

cout out out2 540u

;capacitor value

resr out2 0 0.01

;capacitor ESR

*3703 internals

emod mod 0 value = {57*v(comp)}

;3703multiplier

vstim comp 0 0 ac 1

;ac stimulus

.ac dec 100 1k 1meg

.probe

.end

With the gain/phase plot in hand, a loop crossover fre-

quency can be chosen. Usually the curves look something

like Figure 11. Choose the crossover frequency in the

rising or flat parts of the phase curve, beyond the external

LC poles. Frequencies between 10kHz and 50kHz usually

work well. Note the gain (GAIN, in dB) and phase (PHASE,

in degrees) at this point. The desired feedback amplifier

gain will be -GAIN to make the loop gain at 0dB at this

frequency. Now calculate the needed phase boost, assum-

ing 60

° as a target phase margin:

BOOST = – (PHASE + 30

°)

If the required BOOST is less than 60

°, a Type 2 loop can

be used successfully, saving two external components.

BOOST values greater than 60

° usually require Type 3

loops for satisfactory performance.

Finally, choose a convenient resistor value for R1 (10k is

usually a good value). Now calculate the remaining values:

(K is a constant used in the calculations)

f = chosen crossover frequency

G = 10(GAIN/20) (this converts GAIN in dB to G in

absolute gain)

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